The present invention relates to a control apparatus for a synchronous machine.
In a power system, a power capacitor is frequently used for improving a steady state stability or a transient stability of the power system. For example, the power capacitor is inserted in series fashion in an ultrahigh voltage transmission line to reduce the reactance of the transmission line apparently, whereby the transmission capacity is increased. When the power capacitor is inserted in a power system coupled with a synchronous machine and the capacitance of the power capacitor is large, a transient oscillation component having a frequency f.sub.e given by the following equation occurs in the power system. ##EQU1## That is, the frequency f.sub.e is varied by Xc=reactance of the power capacitor, Xe=reactance of the power system, Xd"=subtransient reactance of the synchronous machine, and other factors not shown. As a result, the transient oscillation component induces a pulsating current with a slip frequency f.sub.o .+-.f.sub.e with respect to the rated frequency f.sub.o of the synchronous machine into the field winding circuit of the rotor and damper winding circuit of the synchronous machine. Normally, such a transient current component is attenuated by the resistive components included in the power system, thus causing no trouble in the power system. In the recent ultrahigh voltage transmission line, however, the resistive components have small values. For this reason, if the power capacitor has a large capacitance, there occurs a case in which the transient oscillation component of frequency f.sub.e is not attenuated but instead diverges. Such instability phenomena of the power system are generally called a self excitation and are considered in one of two categories. The first category is called the induction generator effect and is an instable phenomenon caused with relation to only the power system and the electrical circuit of the synchronous machine. The other is called the sub-synchronous resonance phenomenon (SSR). The SSR is an instable phenomenon including a low frequency resonant oscillation between the power system and the rotatable torsional system including a rotor of a synchronous machine, a turbine and a shaft coupling the rotor and turbine. The SSR is also caused between the power system and the electrical circuit of the synchronous machine. More precisely, when disturbance occurs in the power system of the fundamental frequency f.sub.o and a transient oscillation current component of a frequency f.sub.e, dependent on the capacitance of the power capacitor and the like, additionally flows into the armature circuit of the synchronous machine and a current with a slip frequency f.sub.s (=f.sub.o .+-.f.sub.e) is induced in the rotor circuit of the synchronous machine. The current of f.sub.s acts on the air gap flux of the synchronous machine to produce an electrical torque pulsating with the slip frequency f.sub.s in the synchronous machine. When the pulsating frequency f.sub.s (=f.sub.o -f.sub.e) lower than the rated frequency of the synchronous machine approximates to one frequency f.sub.m of the inherent oscillatory frequencies depending on the torsion of the turbine shaft system and the damping effect to the torsion is insufficient, the electrical torque of the synchronous machine amplifies the torsional oscillation of the turbine shaft system. The torsional oscillation of the turbine shaft system induces a speed voltage of the frequency f.sub.e of the transient oscillation component in the armature circuit of the synchronous machine. If the growing of the torsional oscillation is left as it is, the positive feedback coupling between the electrical system of the synchronous machine and the turbine shaft causes divergent instable phenomenon. This instable phenomenon is the SSR.
The following countermeasures have been taken for decreasing the SSR: (1) minimizing the capacitance of the power capacitor causing the transient oscillation component of the frequency f.sub.e ; (2) inserting a resistor in series to the power transmission system for decreasing the transient oscillation component; (3) removing the transient oscillation component of f.sub.e by inserting a static filter in the power transmission system. Those countermeasures, however, have the following disadvantages. The (1) prevents the improvement of the transient stability in the power transmission system. The (2) increases the power transmission loss. The (3) increases the cost of the equipment. Accordingly, those approaches have insufficiently succeeded in attaining their ends.